F28PL1 Programming Languages Lecture 3: Assembly Language 2

Running assembly programs on the ARM-MDK ARM-MDK does not simulate a bare CPU oriented to embedded systems with memory, peripherals etc STM32-Discovery board is: – STMicroelectronics - STM32F103VB assumes no operating system must provide start up file – code to initialise peripherals, interrupts etc STM32-Discovery uses: – STM32F10x.s –.s == assembler

Running assembly programs on the ARM-MDK start up file associates reset button on board with: ; Reset Handler Reset_Handler PROC EXPORT Reset_Handler [WEAK] IMPORT __main LDR R0, =__main BX R0 ENDP ; text == comment Reset_Handler – labelled procedure start PROC – start of procedure EXPORT Reset_Handler – make Reset_Handler visible externally

Running assembly programs on the ARM-MDK ; Reset Handler Reset_Handler PROC EXPORT Reset_Handler [WEAK] IMPORT __main LDR R0, =__main BX R0 ENDP IMPORT – make external label __main visible internally LDR R0,=__main – load R0 with address for __main BX R0 – branch indirect on R0 i.e. set PC to address for __main from R0 ENDP - end procedure – not a return !

Running assembly programs on the ARM-MDK ARM-MDK expects every project to include a main.c – usually defining a main function – compiler generates assembly with EXPORT ed __main label – start up then IMPORT s __main can’t embed Thumb2 assembly language in C provide a dummy main.c containing only: /* main.c */ GJM’s main.c

Running assembly programs on the ARM-MDK provide own assembly language file with __main : AREA ASMmain, CODE ; Main __main PROC EXPORT __main ENDL B ENDL ENDP END AREA – introduces block of code or data – name + attributes __main – labelled procedure start EXPORT __main – make __main visible externally

Running assembly programs on the ARM-MDK AREA ASMmain, CODE ; Main __main PROC EXPORT __main ENDL B ENDL ENDP END ENDL B ENDL – endless loop at end of program END – end of source file template in GJM’s ASMmain.s

Create new project create a new folder for new project copy dummy main.c to folder copy ASMmain.s template to folder run Keil μVision 4 Project -> New μVision Project – in new folder – give project same name

Create new project Select Device for Target -> STMicroelectronics -> STM32F103VB – OK Copy STM32 Startup Code to Project Folder and Add Project to File? – No Project Window now shows: +Target 1 select + Project Window now shows: +Target 1+Source Group 1

Create new project select inner + Project Window now shows: +Target 1+Source Group 1 Source Group 1 /right mouse button… …Add Files To Group ‘Source Group 1’ Add: STM32F10x.s Add: main.c and (possibly renamed) ASMmain.s – Close

Compile/assemble ASMmain.s /double left mouse button ASMmain.s opens in tabbed window add your assembler code Project -> Build Target – details in lower window fix program and repeat until: “project.axf" - 0 Error(s), 0 Warning(s).

Execute program on simulator Debug -> Start/Stop Debug Session – OK View -> Register Window View -> Disassembly Window select STM32F10x.s tab

Execute program on simulator select reset button: yellow arrow points at: LDR R0,=__main repeatedly select step button: watch indicated instruction & registers changing to leave simulator: Debug -> Start/Stop Debug Session RST

Multiply & divide MUL {Rd, }Rn, Rm  Rd = Rn * Rm UDIV {Rd, }Rn, Rm  Rd = Rn / Rm unsigned rounds down i.e. 7/3 == 2

Expressions for: var 1 = var 2 op var 3 with: var 1 RN R 1 var 2 RN R 2 var 3 RN R 3  op R 1,R 2, R 3 e.g. x = y*z;  x RN 1 y RN 2 z RN 3 MUL x,y,z

Expressions for: var 1 = var 2 op 1 var 3 op 2 var 4 if op 1 and op 2 have same precedence or op 1 has higher precedence than op 2  op 1 R 1,R 2, R 3 op 2 R 1,, R 4 e.g. x = y*z-a; ... a RN 4 MUL x,y,z SUB x,a

Expressions for: var 1 = var 2 op 1 var 3 op 2 var 4 if op 2 has higher precedence than op 1 must evaluate op2 expression in new register  op 2 R i,R 3, R 4 op 1 R 1,, R 2,R i e.g. x = y+z*a;  MUL R5,z,a ADD x,y,R5 e.g. x = y*(z+a)  ADD R5,z,a MUL x,y,R5

Expressions draw expression tree allocate registers to nodes – from bottom left – if expression in assignment then start with register for destination variable accumulate into register for left operand can re-use any register whose value is no longer required

Example: Pythagorus int x; /* R1 */ int y; /* R2 */ int z; /* R3 */ int p; /* R4 */ x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; == + * xx * yy * zz

Example: Pythagorus int x; /* R1 */ int y; /* R2 */ int z; /* R3 */ int p; /* R4 */ x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; == + * R5 xx * yy * zz

Example: Pythagorus int x; /* R1 */ int y; /* R2 */ int z; /* R3 */ int p; /* R4 */ x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; == + * R5 xx * R6 yy * zz

Example: Pythagorus int x; /* R1 */ int y; /* R2 */ int z; /* R3 */ int p; /* R4 */ x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; == + R5 * R5 xx * R6 yy * zz

Example: Pythagorus int x; /* R1 */ int y; /* R2 */ int z; /* R3 */ int p; /* R4 */ x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; == + R5 * R5 xx * R6 yy zz

Example: Pythagoras int x; /* R1 */ int y; /* R2 */ int z; /* R3 */ int p; /* R4 */ x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; == CMP + R5 * R5 xx * R6 yy zz

Example: Pythagoras int x; int y; int z; int p; x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; AREA PYTH, CODE ; Main __main PROC EXPORT __main x RN 1 y RN 2 z RN 3 p RN 4 MOV x,#3 MOV y,#4 MOV z,#5

Example: Pythagoras int x; int y; int z; int p; x = 3; y = 4; z = 5; if(x*x+y*y==z*z) p=1; else p=0; MUL R5,x,x MUL R6,y,y ADD R5,R6 MUL R6,z,z CMP R5,R6 BEQ SAME MOV p,#0 B ENDL SAMEMOV p,#1 ENDLB ENDL ENDP END

Structured programming assembly language requires intricate use of labels & branches easy to produce “spaghetti code” design assembly programs using high level program structures – condition – iteration use template to translate high level to label + branch

Structured programming: if IF exp 1 op exp 2 THEN command 1 ELSE command 2 not( = )  NE not(! = )  EQ not( < )  GE not( <= )  GT not( > )  LE not(> = )  LT Ri = exp 1 Rj = exp 2 CMP Ri,Rj B not(op) IFALSE command 1 B IEND IFALSE command 2 IEND

Structured programming: if IF exp 1 op exp 2 THEN command Ri = exp 1 Rj = exp 2 CMP Ri,Rj B not(op) IEND command IEND

Structured programming: while WHILE exp 1 op exp 2 DO command WLOOP Ri = exp 1 Rj = exp 2 CMP Ri,Rj B not(op) WEND command B WLOOP WEND

Example: is x prime? dreadful algorithm...! int x; int n; int p; x = 23; n = 2; p = 1; while(n<x) if(x%n==0) { p = 0; break; } else n = n+1; AREA PYTH, CODE ; Main __main PROC EXPORT __main x RN 1 n RN 2 p RN 3 MOV x,#25 MOV n,#2 MOV p,#1

Example: is x prime? dreadful algorithm...! int x; int n; int p; x = 23; n = 2; p = 1; while(n<x) if(x%n==0) { p = 0; break; } else n = n+1; TESTCMP n,x BEQ ENDL UDIV R4,x,n MUL R4,n CMP R4,x BNE NOTP MOV p,#0 B ENDL NOTPADD n,#1 B TEST ENDLB ENDL ENDP END n<x x%n==0  (x/n)*n==x n = n+1 p=0; break